Accelerometer



Patented Jan. 2, 1951 ACCELEROMETER Robert 0. Fehr, Augustus n. Fiske.in, and Richard J. Wells, Schenectady, N. Y., asaignors to G neralElectric Company, a corporation of New York Application November 28,1948, Serial No. 62,122

1 Claim. (Cl. 171-327) This invention relates to an improvedaccelerometer, and in particular to an accelerometer adapted to measureshock waves traveling through the ground as a result of largeexploslons.

An object of the invention is to provide an accelerometer of the typedescribed which has high sensitivity, and at the same time is simple andrugged in construction. Other objects and advantages will appear as thedescription proceeds.

The features of the invention which are believed to be novel andpatentable are pointed out in the claim forming a part of thisspecification. For a better understanding of the invention, reference ismade in the following description to the accompanying drawing, in whichFig. 1 shows a vertical section of a preferred embodiment of theinvention; Fig. 2 shows a section along the line 2-2, Fig. 1; Fig. 3 isa view showing in greater detail the construction of the crystalassembly and its attachment to the accelerometer post; and Figs. 4 and 5are circuit diagrams showing electrical connections to the crystals.Where the same part appears in more than one figure of the drawing, itis represented in each case by the same reference numeral.

Referring now to Figs. 1 and 2, the accelerometer includes a base I, anda massive unitary post 2 which is rigidly attached at one of its ends tothe base. Preferably, base I and post 2 are machined from a single pieceof metal, and post 2 has a uniform square cross section throughout itslength. A cylindrical bore extends along the axis of the post, and aco-axial cable 3 is fitted within this bore. A gasket 4, washer 5, andnut 6 provide a moisture-tight seal between the cable and base I. Acover I and gasket 8 complete the moisturetight assembly so that allworking parts are protected from moisture and other disturbinginfluences.

Thin piezoelectric crystals 8 and III are attached to opposite sides ofpost 2 near the base, as shown. An insulating sheath I I, which may be 7of paper but preferably is an impermeable thermosetting plastic, isprovided about the post to electrically insulate it from the crystals.Electrical connections from the crystals to the coaxial cable are madethrough a terminal board It. The inner conductor of the co-axial cableis represented at I3 and the outer conductor at ll.

Refer now to Fig. 3, which shows in greater detail the construction of acrystal assembly and its attachment to the accelerometer post. Crystal 8may be an ADP crystal approximately ten mils thick, flve-eighths inchlong. and the same width as post 2. Such a crystal has been form! togive good results with a steel post having a one inch square crosssection. In'contact with and substantially covering the two large facuof crystal 9 are two electrodes II and it. These electrodes may beformed by depositing a thin coating of metal on the large faces of thecrystal. The entire assembly is held, together, and one large face ofthe crystal is rigidly attached to post 2, by a strong glue, cement, orother bonding agent. An alkyd resin cement has been found satisfactoryfor this purpose. Leads I1 and I! are respectively connected to the twoelectrodes, as shown. 4

Refer now to Figs. 4 and 5, which are circuit diagrams of twoalternative electrical connections. The two crystals may be connected inseries, as in Fig. 4, or in parallel, as in Fig. 5, with theirpolarities opposed. For the series connection, as shown in Fig. 4, ifthe two crystals are deformedin the sarne direction, as by tension orcompression of the accelerometer post, the outer face of each crystalbecomes positive with respect to the face adjacent post 2, so that thevoltages across the two crystals are of opposite polarity inthe circuitand balance out. For the parallel connection shown in Fig. 5, one of thecrystals is reversed so that when they are deformed in the samedirection the outer face of one has the same polarity as the inner faceof the other. The final result is the same, but with the parallelcircuit it is current produced by the opposing voltages rather than thevoltages themselves which balance out.

The accelerometer is responsive to accelerating forces normal to theaxis of post 2 and the plane of crystals 9 and I0. For example, whenused as a ground wave detector. the accelerometer may be installed sothat any shock waves traveling through the ground will be transmitted tobase I. Assume that these shock waves are from left to right in theplane of the section shown in Fig. 1. As base I is acceleratedby theshock waves, the inertia of post 2 produces stresses which slightly bendthe post. In other words. the post acts as a cantilever beam which isdeflected by the inertia of its own mass in response to accelerationnormal to its axis. It should be noted that in this accelerometer, theinertia mass is distributed throughout the length of the post. In thisrespect, the accelerometer differs from the lumped-mass. type in whichthe inertia mass acts as a unit upon a spring or other resilient member.

when post 2 bends, one side of the post is placed in tension and theother side in compression. This mechanically deforms crystals I and I0respectively in opposite directions, that is, one of the crystals isshortened in a direction parallel to its large faces, while the othercrystal is lengthened in the same direction. Voltages of oppositepolarities are thus produced between the large faces of the respectivecrystals, but since the crystals are connected in the electrical circuitwith their polarities opposed, these voltages add together and producean output voltage of double value between conductors i3 and ll of theco-axial cable. The other end of cable 3 is connected to suitable.voltage-responsive apparatus, such as an amplifier and an-indicating orrecording instrument, which may produce an indication proportional tothe voltage between conductors i 3 and It, and hence to the magnitude ofthe acceleration measured.

Acceleration in other than the desired direction has negligible effectupon the output voltage. For example, acceleration in the direction ofthe axis of post 2 produces tension or compression simultaneously onboth sides of the post. In this case, the voltages produced acrosscrystals 9 and ill have polarities such that the two voltages baanceout. as hereinbefore explained, and consequently no net voltage isproduced between conductors l3 and II responsive to such acceleration.

Having described the principle of this invention and the best mode inwhich we have contemplated app'ying that principle, we wish it to beunderstood that the apparatus described is illustrative only, and thatother means can be employed without departing from the true spiritand'scope. oi the invention defined by the following claim.

What we claim as new and desire to secure by Letters Patent oi theUnited States is:

' assaeos rigidly attached at one of its ends to the base so that thepost deflects as a cantilever beam responsive to acceleration normal toits axis, said post having a. substantially uniform square cross sectionthroughout its length, a coaxial cable, the post and base having acylindrical bore extending along the axis of the post into which thecoaxial cable fits, thinpiezoelectric crystals respectively attached toopposite sides of said post near the base, each such crystal having tworelatively large opposite faces one of which is rigidly attached to aside of the post, electrodes in contact with and substantially coveringthe large faces of the crystals, an insulating sheath about the post toelectrically insulate it from the crystals and their electrodes, andelectrical connections between electrodes and the co-axial cable suchthat crystals on opposite sides of the post are connected with opposedpolarities.

ROBERT O. FEHR. AUGUSTUS .H. FIBKE, Jn. RICHARD J. WELLS.

REFERENCES CITED The following references are 01 record in the idle 01'this patent:

UNITED STATES PATENTS Number Name Date 1,693,806 Cady Dem- 4, 19281,912,213 Nieolson May 30,; 1933 2,266,449. Ullrich et al Dec. 16, 19412,359,245 Ritzmann Sept. 26, 1944 2,371,626 Keckemetl Mar. 20, 1945

